1. Field of the Invention
The present invention relates to a high tenacity, high elongation, low shrinkage synthetic fiber treated to enhance stiffness and interfilament coherency, and to improve partitioning properties. The resulting fiber, combined in multiple high denier bundles, can be cut at high speed to suitable reinforcing lengths for use in reinforcing a resin matrix. The cut fiber can be used in bulk molding compounds (BMC), sheet molding compounds (SMC) and spray-up applications due to ease of cutting.
2. Description of Related Art
Synthetic fibers in staple or filamentary form, and fabrics produced therefrom, are known for polymer reinforcement. Typical of the fibrous reinforcements are glass, polyester, polyamide (nylon and aramid) and polyolefin fibers. Conventional matrix resins include thermoplastics, such as nylon and polyolefins, and thermosetting materials, such as epoxy and unsaturated polyester resins. Since the primary function of the interface between fiber and matrix is to transmit stress from the matrix to the reinforcing fibers, the chemical and physical features of the interface are critical to the mechanical properties and end use performance of the composite. The compatibility between the reinforcing fiber and matrix is then a determining factor in the load sharing ability of the composite.
Fiber coatings/binders have been used to enhance the compatability of the reinforcing fibers and resins with which they are to be used.
Fiber bundle integrity is the degree to which individual filaments adhere to each other. A benefit of a low integrity fiber is that it enables good dispersion of single filaments throughout a resin matrix. This even distribution results in a homogeneous reinforced composite, a direct result of which is an improved cosmetic appearance. U.S. Pat. No. 4,632,864 to Cordova et al., issued Dec. 30, 1986, discloses a composite reinforced with a high tenacity reinforcing fiber with low bundle integrity, the fiber having been treated with a composition comprising a vinyl chloride copolymer having a glass transition temperature of greater than about 60.degree. C. to enhance compatibility of the fiber and the resin matrix.
However, in operations where high speed cutting, on the order of from 100 to 500 feet per minute (about 30 to 150 meters per minute) or higher, of the fiber is required, e.g., SMC's and spray-up, a low integrity fiber tends to fluff or cottonball and jam up the cutter. In such applications a high integrity yarn that is readily cut at high speed, that does not ball-up within the reinforced composite, and that provides individual bundle separation is desirable.
U.S. patent application Ser. No. 136,080 filed Dec. 21, 1987, discloses a composite reinforced with a high tenacity reinforcing fiber with high bundle integrity, the fiber having been treated with a composition comprising an aqueous solution of carboxyl-terminated, oil-free alkyd resin which is the reaction product of at least one aliphatic glycol containing 2 to 12 carbons with a combination of aromatic di- or trifunctional carboxylic acids and, optionally, unsaturated aliphatic acids. These alkyd resins are polymerized below their gel points, that is the degree of esterification is generally maintained below about 90 percent. Bundle integrity is increased while retaining compatibility of the reinforcing fiber with the resin.
In addition to advantages in high speed cutting, a fiber with high bundle integrity offers advantages in those applications in which it is preferred that the bundle of filaments remain coherent in the resin matrix rather than to disperse into single filaments. A finish such as that discussed above in U.S. patent application Ser. No. 136,080 serves to increase bundle integrity which is preserved when the cut fiber is mixed with resin matrix. However, the finish required for bundle integrity can create storage problems for the reinforcing fiber. If several ends of multifilament yarn are combined into tow, then rolled into a package, pressure during storage may be sufficient to cause the several ends to adhere together, creating problems in the separation of these ends into individual bundles. If a tow is produced (a number of fiber bundles drawn together for ease of cutting in SMC and BMC), the tow must separate into distinct fiber bundles (ends) such that clumps of yarn do not occur. This is necessary for homogeneous properties of the reinforced composite and improved cosmetic appearance. For instance, filaments from a first end may adhere too tightly to a second end and peel away from the first end when the second end is separated. Such "ribboning" is undesirable. Another example of "ribboning" would occur during unwinding, when individual filaments from beneath may adhere to the layer being unwound, causing entanglements which can result in breaking of the fiber. Also, if the multifilament yarn is first cut to reinforcing lengths, then packed and stored, the pressure during storage may cause the individual cut bundles to "clump" together, creating problems when the cut fiber is mixed into the resin matrix.
Thus, the need exists for synthetic fiber for reinforcing plastic composites, said fiber having high bundle integrity to permit high speed cutting and unwinding and to permit inclusion of a coherent bundle of filaments into the resin matrix, yet providing sufficient properties that the individual bundles of filaments do not adhere to each other.